NOTES- API EXAMS-PRATHAP DHAS

Figure 3-38 illustrates the squeezing and distortion of flux fields at the start and finish of a seam weld. At
the start, the magnetic flux lines are concentrated behind the electrode. The arc tries to compensate for
this imbalance by moving forward which creates forward arc blow. As the electrode approaches the end of
the seam, the squeezing is ahead of the arc, with a resultant movement of the arc backwards, and the
development of back blow. At the middle of a seam in two members of the same width, the magnetic field
would be symmetrical, and there would not be any back or forward arc blow. But, if one member should
be wide and the other narrow, side blow could occur at the midpoint of the weld.

Understanding the Effect of Welding Current Returning through the Work piece
Another "squeezing" phenomenon results from the current returning back towards the work piece
connection within the work piece. As shown in Figure 3-39, a magnetic flux is also set up by the electrical
current passing through the work piece to the work piece lead. The heavy line represents the path of the
welding current while the light lines represent the magnetic field set up by the current. As the current
changes direction, or turns the corner from the arc to the work, a concentration of flux occurs at x, which
causes the arc to blow, as indicated, away from the work piece connection.
NOTES- API EXAMS-PRATHAP DHAS

The movement of the arc because of this effect will combine with the movement resulting from the
concentration previously described to give the observed arc blow. The effect of the returning current may
diminish or increase the arc blow caused by the magnetic flux of the arc. In fact, control of the direction of
the returning current is one way to control arc blow, especially useful with automatic welding processes.

In Figure 3-40(a), the work piece cable is connected to the starting end of the seam, and the flux resulting
from the returning welding current in the work is behind the arc. The resulting arc movement would be
forward. Near the end of the seam, however, the forward arc movement would diminish the total arc blow
by canceling some of the back blow resulting from concentration of the flux from the arc at the end of the
work piece, see Figure 3-41(a). In Figure 3-40(b), the work cable is connected to the finish end of the
seam, which results in back blow. Here, it would increase the back blow of the arc flux at the finish of the
weld.
NOTES- API EXAMS-PRATHAP DHAS

The combination of "squeezed" magnetic fluxes is illustrated in Figure 3-41(b). A work piece connection
at the finish of the weld, however, may be what the welder needs to reduce excessive forward blow at the
start of the weld.

Because the effect of welding current returning through the work piece is less forceful than concentrations
of arc-derived magnetic flux at the ends of workpieces, positioning of the workpiece connection is only
moderately effective in controlling arc blow. Other measures must also be used to reduce the difficulties
caused by arc blow when welding.

Other Problem Areas

Corner and Butt Joints with deep Vee grooves

Where else is arc blow a problem? It is also encountered in the corners of fillet welds and in weld joints
which use deep weld preparations. The cause is exactly the same as when welding a straight seam -
concentrations of lines of magnetic flux and the movement of the arc to relieve such
concentrations. Figures 3-42 and 3-43 illustrate situations in which arc blow with DC current is likely to
be a problem.

High Currents
There is less arc blow with low current than with high. Why? Because the intensity of the magnetic field a
given distance from the conductor of electric current is proportional to the square of the welding current.
Usually, serious arc blow problems do not occur when stick electrode welding with DC up to
approximately 250 amps (but this is not an exact parameter since joint fitup and geometry could have
major influence.)

DC Currents
The use of AC current markedly reduces arc blow. The rapid reversal of the current induces eddy currents
in the base metal, and the fields set up by the eddy currents greatly reduce the strength of the magnetic
fields that cause arc blow.

Magnetically Susceptible Materials

Some materials, such as 9%nickel steels, have very high magnetic permeability and are very easily
magnetized by external magnetic fields, such as those from power lines, etc. These materials can be very
difficult to weld due to the arc blow produced by the magnetic fields in the material. Such fields are easily
detected and measured by inexpensive hand - held Gauss meters. Fields higher than 20 Gauss are
usually enough to cause welding problems.
NOTES- API EXAMS-PRATHAP DHAS

Thermal Arc Blow

weve already examined the most common form of arc blow, magnetic arc blow, but what other forms
might a welder encounter? The second type is thermal arc blow. The physics of the electric arc require a
hot spot on both the electrode and plate to maintain a continuous flow of current in the arc stream. As the
electrode is advanced along the work, the arc will tend to lag behind. This natural lag of the arc is caused
by the reluctance of the arc to move to the colder plate. The space between the end of the electrode and
the hot surface of the molten crater is ionized and, therefore, is a more conductive path than from the
electrode to the colder plate. When the welding is done manually, the small amount of "thermal back
blow" due to the arc lag is not detrimental, but it may become a problem with the higher speeds of
automatic welding or when the thermal back blow is added to magnetic back blow.

Arc Blow with Multiple Arcs

Some recent welding process advances involve the use of multiple welding arcs for high speed and
improved productivity. But, this type of welding can also cause arc blow problems. Specifically, when two
arcs are close to each other, their magnetic fields react to cause arc blow on both arcs.

When two arcs are close and have opposite polarities, as in Figure 3-44(a), the magnetic fields between
the arcs causes them to blow away from each other. If the arcs are the same polarity, as in Figure 3-
44(b), the magnetic fields between the arcs oppose each other. This results in a weaker field between the
arcs, causing the arcs to blow toward each other.
NOTES- API EXAMS-PRATHAP DHAS

Usually, when two arcs are used, it is suggested that one be DC and the other AC, as shown in Figure 3-
44(c). In this case, the flux field of the AC arc completely reverses for each cycle, and the effect on the
DC field is small. As a result, very little arc blow occurs.

Another commonly used arrangement is two AC arcs. Arc blow interference here is avoided to a large
extent by phase-shifting the current of one arc 80 to 90 degrees from the other arc. A so-called "Scott"
connection accomplishes this automatically. With the phase shift, the current and magnetic fields of one
arc reach a maximum when the current and magnetic fields of the other arc are at or near minimum. As a
result, there is very little arc blow.

How To Reduce Arc Blow

Not all arc blow is detrimental. In fact, a small amount can sometimes be used beneficially to help form
the bead shape, control molten slag, and control penetration. When arc blow is causing or contributing to
such defects as undercut, inconsistent penetration, crooked beads, beads of irregular width, porosity,
wavy beads, and excessive spatter, it must be controlled.

Possible corrective measures include the following:

If DC current is being used with the shielded metal-arc process - especially at rates above 250 amps - a
change to AC current may eliminate problems

Hold as short an arc as possible to help the arc force counteract the arc blow

Reduce the welding current - which may require a reduction in arc speed

Angle the electrode with the work opposite the direction of arc blow, as illustrated in Figure 3-45

Make a heavy tack weld on both ends of the seam; apply frequent tack welds along the seam, especially
if the fitup is not tight

Weld toward a heavy tack or toward a weld already made

Use a back-step welding technique, as shown in Figure 3-46

Weld away from the workpiece connection to reduce back blow; weld toward the workpiece connection to
reduce forward blow

With processes where a heavy slag is involved, a small amount of back blow may be desirable; to get
this, weld toward the workpiece connection

Wrap the work cable around the workpiece so that the current returning to the power supply passes
through it in such a direction that the magnetic field set up will tend to neutralize the magnetic field
causing the arc blow
NOTES- API EXAMS-PRATHAP DHAS

The direction of the arc blow can be observed with an open-arc process, but with the submerged arc
process it is more difficult to diagnose and must be determined by the type of weld defect.

Back blow is indicated by the following:

Spatter
Undercut, either continuous or intermittent
Narrow, high bead, usually with undercut
An increase in penetration
Surface porosity at the finish end of welds on sheet metal

Forward blow is indicated by:

A wide bead, irregular in width

Wavy bead
Undercut, usually intermittent
A decrease in penetration

The Effects of Fixturing on Arc Blow

Another precaution the weld operator needs to be aware of with arc blow is its relationship to fixturing.
Steel fixtures for holding the workpieces may have an effect on the magnetic field around the arc and on
arc blow and may become magnetized themselves over time. Usually, the fixturing does not cause any
problems with stick-electrode welding when the current does not exceed 250 amps. Fixtures for use with
higher currents and with mechanized welding should be designed with precautions taken so that an arc
blow-promoting situation is not built into the fixture. Each fixturing device may require special study to
ascertain the best way to prevent the fixture from interfering with the magnetic fields.

The following are some points to note:

Fixtures for welding the longitudinal seam of cylinders (see Figure 3-47) should be designed for a
minimum of 1-inch clearance between the supporting beam and the work. The clamping fingers or bars
that hold the work should be nonmagnetic. Do not attach the workpiece cable to the copper backup bar;
make the work connection directly to the work piece if possible

Abricate the fixture from low-carbon steel. This is to prevent the buildup of permanent magnetism in the
fixture

Welding toward the closed end of "horn type" fixtures reduces back blow
NOTES- API EXAMS-PRATHAP DHAS

Design the fixture long enough so that end tabs can be used if necessary

Do not use a copper strip inserted in a steel bar for a backing, as in Figure 3-48. The steel part of the
backup bar will increase arc blow

Provide for continuous or close clamping of parts to be seam-welded. Wide, intermittent clamping may
cause seams to gap between clamping points, resulting in arc blow over the gaps

Do not build into the fixture large masses of steel on one side of the seam only. Counter-balance with a
similar mass on the other side

By understanding the mechanics of arc blow and how to correctly diagnose it in the weld, operators
should be able to eliminate it from their applications and be able to create welds without the problems
normally associated with arc blow.